Alpha-olefins are made in commercial quantities by a process initially developed in the fifties by Karl Ziegler and his co-workers. The so-called Ziegler process involves the reaction of triethyl aluminum ("TEA") and ethylene at temperatures in the range of 200-500.degree. F. and pressure in the range of 2000-5000 psig to yield a mixture of tri-C.sub.2-20+ alkyl aluminum having a poisson alkyl distribution and C.sub.2-20 olefins. The ethylene is flashed from the reaction mixture for recycle and the light olefins through decene-1 can be distilled from the mixed aluminum alkyls since they have a normal boiling point below the lightest aluminum alkyl (viz. TEA).
Johnson, U.S. Pat. No. 2,863,896, describes the preparation of pure aluminum alkyls using a chain growth reaction of a C.sub.2-4 olefin (e.g. ethylene) with a low molecular weight trialkyl aluminum (e.g. TEA), dialkyl aluminum hydride or alkyl aluminum dihydride. The chain growth product contained about 2-5 percent C.sub.4-20 olefin which could not be separated from the aluminum alkyls. The mixture was then subjected to a displacement reaction with a C.sub.4-6 .alpha.-olefin, e.g. 1-butene, to displace mainly C.sub.6-20 .alpha.-olefin forming tributyl aluminum. The C.sub.6-20 .alpha.-olefins were fractionated into individual .alpha.-olefins. These individual .alpha.-olefin cuts were then reacted in a second displacement reaction with the tributyl aluminum formed in the butene displacement reaction to form pure trialkyl aluminum.
Catterall et al., U.S. Pat. No. 2,889,385, describe an ethylene chain growth reaction carried out on tributyl aluminum followed by displacement with 1-butene to regenerate tributyl aluminum and evolve C.sub.4-20 .alpha.-olefins. This avoids the problem encountered in attempting to separate TEA from C.sub.12-14 .alpha.-olefins which have normal boiling points close to the same temperature.
Other patents disclosing variations of the aluminum alkyl chain growth .alpha.-olefin synthesis are U.S. Pat. No. 2,906,794; U.S. Pat. No. 2,971,969; U.S. Pat. No. 3,180,881; U.S. Pat. No. 3,210,435; U.S. Pat. No. 3,227,773; U.S. Pat. No. 3,278,633; U.S. Pat. No. 3,352,940; U.S. Pat. No. 3,663,647; U.S. Pat. No. 3,789,081; U.S. Pat. No. 3,487,097; U.S. Pat. No. 4,314,090; U.S. Pat. No. 3,359,292; U.S. Pat. No. 3,384,651; U.S. Pat. No. 3,415,861; U.S. Pat. No. 3,458,594 and U.S. Pat. No. 3,358,050.
All aluminum alkyl chain growth products initially yield a mixture of higher trialkyl aluminum compounds exhibiting a poisson distribution. When ethylene and TEA are used, the mixture is mainly tri-C.sub.2-20 alkyl aluminum compounds although a small amount of C.sub.20+ alkyls are usually present. Ethylene displacement of this mixture yield a mixture of C.sub.2-20 olefins. The more valuable components are the C.sub.6-14 .alpha.-olefins. Ethylene can be recycled to chain growth and olefins above C.sub.14 can be separated for use as diluent or can be purged. Butene is produced in fairly large amounts and cannot economically be discarded. The commercial market for butene is substantially saturated. The present invention provides a process that utilizes the butene that it produces in a closed butene displacement loop that gives high yield of the more desirable C.sub.6-12 .alpha.-olefins.